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You'll need to specify your system specification, especially the motherboard. And while you're at it, what you're intending to do with your machine, i.e. gaming or office work.
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happy_soilAug 15 '13 at 9:54

3

Download the manual for the motherboard. Intel also publishes a memory compatability for their platform.
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RamhoundAug 15 '13 at 10:55

2 Answers
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According to Crucial you have two slots, each that can take a 8GB max stick of PC3-12800 (DDR3-1600) RAM for a max total of 16GB of RAM. However, Intel lists support for up to 16GB of DDR3 1333 MHz which is PC3-10600.

Currently you have a single stick of 2GB PC3-PC3-10666 (DDR3-1333) and conventional wisdom is to match the speed (though go for a higher capacity if you can).

You mention you have a Sandy Bridge i3 CPU and you are looking for better performance. If you listed more specs and what you goal is that would help, though I suspect if you upgraded the CPU to an i5 or i7 (and Ivy Bridge CPUs are supported by the H61 chipset; you wouldn't gain native USB3.0 support and other features though) you'd see a substantial performance gain.

If you do decide to upgrade your processor, I'd strongly recommend a bios update to the latest version available before you do a CPU swap, since you may need a update to support the new chip
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Journeyman Geek♦Aug 15 '13 at 10:26

"Fractional frequencies are normally rounded down, but rounding up to 667 is common due to the exact number being 666⅔ and rounding to the nearest whole number. Some manufacturers also round to a certain precision or round up instead. For example, PC3-10666 memory could be listed as PC3-10600 or PC3-10700" source Wikipedia
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justbrowsingAug 15 '13 at 22:58

According to Intel's specifications for your particular motherboard, it supports dual-channel memory on two memory banks. The current memory module fitted is 2GB rated at 10700 MHz (667 MHz operational), with 9-9-9-24 (CL9) timings.

What you can do is buy another memory module that has the same specifications as the one that you've got (2GB, 9-9-9-24/CL9, PC3-10700 or something similar) so you'll end up having 4GB of RAM. Since you're going to be using your system for development and gaming work, your best bet would be discarding your current 2GB module and instead going for an 8GB memory kit (that is, 2 x 4GB of RAM). Specs for the kit should be roughly the same, apart from the capacity (try to avoid the more expensive options). That way you'll have a bit of leeway with running multiple programs simultaneously and also improving overall system performance.